Thanks Merlin.
So,.........if I'm going to play around with CD's for astronomical spectra, the ''reflective'' method would be the way I presume?
I don't know what type of gratings the R.A and Rigel Systems use, but I've found that the direct view spectra with CD's or DVD's seems to be way off to the edge of the grating and not close to the image of the star.
What is the reason for this I wonder?
The Rigel uses a 600 l/mm grating.
The deviation angle ( between the zero order and the spectrum) is roughly 3.8 degrees per 100 l/mm; so for the CD at 630 l/m that's almost 23 degrees and double that again for a DVD.
Here is a program with instructions to make an effective spectrascope. It was originally supplied by Meade to use with a 416 CCD on a LX series telescope. The program is shareware but you can try it for free. It is compiled basic so may not run on a 64 bit computer but runs OK on my XP.
You will need a CCD camera or similar and a good laser printer that can print the grating onto overhead projector film. I successfully printed out a 200 line/inch grating with a HP 5p laser printer but the overhead paper won't go through through my Konica
Unzip the program read the manual and look at the star spectra
Here is a program with instructions to make an effective spectrascope. It was originally supplied by Meade to use with a 416 CCD on a LX series telescope. The program is shareware but you can try it for free. It is compiled basic so may not run on a 64 bit computer but runs OK on my XP.
You will need a CCD camera or similar and a good laser printer that can print the grating onto overhead projector film. I successfully printed out a 200 line/inch grating with a HP 5p laser printer but the overhead paper won't go through through my Konica
Unzip the program read the manual and look at the star spectra
Barry
Wow, just saw your post Barry!
Thanks for that!
This will be interesting to say the least. I'll give it a try
barry,
At 100 or 200 lines per inch (4 to 8 l/mm) that's a very low line count.
More like a Ronchi grating than a diffraction grating. At 8 l/mm the resolution will be very poor and only macro detail will be observed in the spectrum.
A Cokin B40 "Cosmos" filter would be better, it is a 240 l/mm grating.
Yes.............................. .CD reflective grating experiments on table top with small light source are looking very, very good
Being an incredibly stubborn and persistant D.I.Y.er, I think I may be on to something here
Going back to the cheap cardboard spectroscope article, I've learnt some important rules on angles, viewing angle amongst other things.
If a crudely made spectroscope (I cheated with an existing small cardboard box) can show a fluro light pin hole spectra, then a star through a telescope should also work............I hope?...........or do I need a slit, collimator, etc.?
Thanks everyone, but it's too much money for me at the moment.
May have to go for the Rainbow Optics gratings for the time being by the looks of it.
Hi Robz,
PM me, I have links to everything you need real cheap (I mean real cheap not shady ). I'm not a seller or affiliated with one. Just know of a good supplier.
mmmm....I don't know why I did not mention this before
I have a thread on a DIY LCD Digital Optical Filter Project in the ATM/DIY section. It uses a LCD screen to filter wavelengths of visible light. The software could incorporate a full spectral scan and in a sense turn it into a digital spectroscopic analyser! If your a little adventurous, you guys should check it out.
here's the thread mentioned... http://www.iceinspace.com.au/forum/s...ad.php?t=72640
Astroguy,
I'm interested in your experiments. I'm assuming that a conventional LCD screen is made up from individual pixels which emit either green, red or blue...these are then "combined" (?) through a bayer matrix solution to display any one of the 64 million colours...Is this correct?
If so, a couple of comments:
What's the size of the individual pixel? What's the average distance between centres?
In a DSLR the De-bayering process actually "manufactures" a virtual pixel at the intersections based on the mathematical model used ( Craig Stark has a very good write-up on his site) and it's this "virtual" coloured pixel you see in the viewfinder and RAW image of a DSLR.
I don't know how the individual pixels (if they are only RGB) in a LED can be made to emit a different colour...so it may be a combination of varying intensity for each pixel giving the "impression" of colour ie combined by our eye....
It is certainly possible to easily test the outcome. We have access to spectroscopes with bandwidth resolution of 0.01nm......
How can we help each other???
Hi
I am not an expert on displays but there are a number of different ways of displaying information on a screen to excite the eye to a respose. I will just look at two types The LCD and the LED screen.
The LCD screen uses a matrix of three filters arranged as three "stripes" of colour filters arranged in a square. This forms one pixel. Put a magnifying glass to the screen and you will see this formation. These filters allow a measured amount of light of each of the primary colours to pass through according to the digitised amount of each colour, illuminated from behind by a white electro lumescent screen. I am not sure of this I think LCD uses CMY as the colours because they filter and LED wil use RGB because they emit but it could be the other way around.
The LED screen emits light in the three colours in the same way but each LED is its own source of light of which the intensity is controlled by the digitised control signal as before.
The eye of course perceives these tri colour spots as a single colour which has as many different tones as the bits of the digitised signal allow.
When you are observing a spectum of a star for instance the actual colour of the various sections will depend mainly on how your screen is set up. The aim here is not the actual perceived colour but the position and number of dark lines that appear on the spectrum corresponding to the absorption spectra of the elements in that star that define its construction and its relative speed.
Barry
Last edited by Barrykgerdes; 07-03-2011 at 06:38 PM.
Barry,
thanks for that...It's sometimes amazing the things we take for granted..
Looking at my laptop screen with a x30 loupe certainly shows the blue/ green/ red elements.
For the purposes of the discussion I'd still call these individual "elements" a pixel....the same as in a colour camera and the "combined" set of three a "picture box" ???
It appears under magnification, that "RED" is represented by the red pixel, some low intensity in the green pixel and zero intensity (ie black!) in the blue...so there's no de-bayering as such ie each pixel is ALWAYS RGB (In a digital image if you do the same and zoom in on the individual pixels ie Maxim/ AstroArt etc each "pixel" on the screen is one of the 64 million colours - but if you then look at each image "pixel" you see on the laptop screen it is still a collage of RGB screen "pixels")
So the size of the screen pixels will obviously define the resolution seen by the eye..
Summary:
Using a screen matrix to generate a "filter" -
A GREEN filter would be a bright green pixel and black in the other two...similarly for the others.
A non RGB "filter" would have some intensity in each pixel.....
I think the issue may be the colour of the object/ spectrum going through the "filter" if it changes quickly from one point to the other ie across a couple of "pixels" there could be colour distortion...
Certainly well worth a trial!!
Consider other options about eg: professional grade diffraction gratings and create the spectroscope shell based on dimensions. Cut the CD or DVD to the required size compared to commercial DG's and insert. If it works (great) if not you have a shell ready to insert a professional grade diffraction grating in.
Personally a CD is round and if you look carefully the data does alter the surface diffraction. this would alter the final diffraction. Being round I am not sure if the colours would reflect accordingly too, blanks may be uniform. I do not know how the the disc are created.
Sorry Robert....back on track....
A CD/DVD is a play thing...IMHO you can't use them for any semi-serious spectroscope work.
A 45 degree prism or better a 60 degree prism will work very well...only one bright spectrum, no zero order image, no multiple spectral orders...
Only down side is low resolution and non-linear dispersion.
(To quote " A 100 l/mm grating at a distance of 100mm will give approximately the same plate scale as a 30mm 60 degree flint prism")
malcolm,
It's almost impossible ( a word I don't use often) to cut a transmission grating without damaging it!
You can make up a mounting frame to hold the 35mm sized filters but this adds backfocus problems and complexity.
There's a design shown in the Y! group astronomical_spectroscope files area that I used with the P-H commercial 35mm gratings...
I have a nice little spectroscope that was loaned to me on a long term loan some years ago. It was essentially for examining liquids. There was a minature test tube that fits in one of the holes. It has an adjustable slot, prism and viewing eyepiece all built in.
Looking at a white TV screen LCD gives colour lines in the blue, red and green areas of the spectrum. Looking at daylight from the sun gives the standard white light spectrum view showing the absorption lines, principally the sodium line. Looking at one of those energy saving floros shows why you should never use them for serious viewing of coloured objects.
Barry
Last edited by Barrykgerdes; 07-03-2011 at 06:39 PM.
.....................no wonder!!!
............... .CD reflective grating experiments on table top with small light source are looking very, very good
). I'm not a seller or affiliated with one. Just know of a good supplier.
